1. Field of the Invention
This invention relates to optical fibers. More particularly, it relates to a method for forming optical fibers and the optical fiber formed thereby.
2. Prior Art
Optical fibers typically include an optical energy transmitting core having a selected index of refraction surrounded by a cladding having an index of refraction less than that of the core to enable guided optical energy transmission through the core from an input port to an output port. There are several common methods for manufacturing optical fibers. The most common method uses a glass starting cylinder or rod, typically high-purity fused silica or doped fused silica, that is coated on its exterior surface with a layer or deposit of a glass or doped glass. The exterior layer can be formed by applying minute soot-like glass or doped glass particles to the rod by well known chemical vapor deposition techniques. The soot-like glass coating is sintered during this application, and the composite structure is then heated to its drawing temperature and drawn down in one or more drawing stages to form the resulting optical fiber with the starting rod defining the resulting core and the deposited coating defining the cladding layer. A variation of this method of forming optical fibers involves coating the interior surface of a hollow glass tube, typically high-purity fused silica or doped fused silica, with a layer or deposit of a glass or doped glass. The interior coating can be applied in the manner described above, that is, by applying a deposit of minute soot-like particles by the chemical vapor deposition technique. After the internal coating is applied, the composite structure is heated to sinter the coating and the hollow tube is drawn down to collapse the tube to form a solid, void-free core. The internal coating defines the optical energy transmitting core and is designed with an index of refraction greater than that of the cladding defined by the hollow starting tube.
While the methods described above are well suited for formal optical fibers, great care is required during the chemical vapor deposition stage to deposit coatings having the proper dopant concentrations to provide a resulting glass having the desired index of refraction and optical transmission qualities. The chemical vapor deposition process can be time-consuming, can require considerable expertise, and is vulnerable to contamination.
Efforts have been made in the direction of optical fibers having an air cladding and optical fibers formed from single-glasses, that is, an optical fiber in which the core and cladding are formed from the same glass and yet has a core with an index of refraction relatively higher than that of the cladding. Typically, such optical fibers have been formed by designing core/cladding structures that place the core under a stress relative the cladding to cause a stress-induced birefringence or stress-induced increase in the index of refraction. One such optical fiber structure is disclosed in U.S. Pat. No. Re 28,664 to S. E. Miller. As disclosed therein, a circularly symmetric optical energy transmitting core is supported on two sides within a hollow, cylindrical support tube or jacket so that the core is at least partially surrounded by an air cladding. The equivalent of a central core portion is supported from the outer tube by thin diametrically extending web-like formations.
While such optical fibers utilizing a single material, or a material having a uniform index of refraction throughout in reliance on the lower index of refraction of air in relation to glass, demonstrate potential advantages over conventional core/cladding fibers, the resulting fiber structure is very fragile. Accordingly, the likelihood of damage from normal handling and use in an optical communication system is high.